The Flame Cannon is formed from Gora (packaged with Deluxe Megatron) in standard cannon mode with his blades deployed forming the core, with Gul (packaged with Starscream) and Noji (packaged with the Vehicon) in weapon modes on opposite sides.
Gora II (packaged with Megatron Darkness) used in place of Gora will still make a Flame Cannon, as revealed on a chart of Combo Weapons released by TakaraTomy. Since the Flame Cannon is only a Combo Weapon made up of mixed Energon Crystal types, you can substitute in Shadow Gora if you like.
The Flame Cannon was revealed on the Vehicon's box-back... where it shows Gul on the bottom of Gora's cannon-mode, and Noji on the top. The configuration pictured here is how Noji's instructions show it assembled.
This weapon (using the original Gora) makes up the entirety of the Vehicon's Arms Up Mode. Really. That's it.